As an apparatus for adjusting the height position of a steering wheel according to the size and driving posture of a driver, there is a tilt steering apparatus such as disclosed in JP 2009-227181 (A), JP 2010-254159 (A) and JP 2011-121443 (A). FIG. 13 to FIG. 16 illustrate an example of conventional construction of a tilt steering apparatus. This tilt steering apparatus has: a steering shaft 1 that is rotated by the operation of a steering wheel (not illustrated in the figure) that is fastened to the rear-end section (right-end section in FIG. 13) of the steering shaft 1; and a steering column 2 that is supported by and fastened to the vehicle body, and supports the steering shaft 1 on the inside thereof so as to be able to rotate freely. The front-end section (left-end section in FIG. 13) of the steering column 2 is supported by a tilt shaft 4 that is supported by a front vehicle-side bracket 3 that is fastened to the vehicle body (not illustrated in the figure) so as to be able to pivot freely. On the other hand, the middle section of the steering column 2 is supported by a rear vehicle-side bracket 5 that is fastened to the vehicle body such that the height position can be adjusted. In this specification, “front and rear” refers to the front and rear in the direction of travel of the automobile.
The rear vehicle-side bracket 5 is obtained by bending a metal plate, and has: an installation-plate section 6 that is provided at the top, and a pair of support-plate sections 7 that are parallel with each other and that hang down from the installation-plate section 6. Long tilt holes 8 that extend in the up-down direction and each have an arc shape centered around the tilt shaft 4 are formed in positions in the support-plate sections 7 that are aligned with each other. Moreover, a displacement bracket 9 having a U-shaped cross-sectional shape and that is obtained by bending a metal plate is fastened by welding or the like to a portion of the middle section of the steering column 2 that is held between the support-plate sections 7. The displacement bracket 9 has a pair of supported sections 10 that are parallel with each other and that overlap with the support-plate sections 7, and through holes 11 that are concentric with each other are formed in portions of these supported sections 10 that are aligned with the long tilt holes 8. A rod member 12 is inserted through these through holes 11 and the long tilt holes 8.
An adjustment lever 14 is provided on one end section in the axial direction (left-end section in FIG. 14) of the rod member 12, and a pressure member 13 is provided on the other end section in the axial direction (right-end section in FIG. 14) of the rod member 12. These members 13, 15 form a tilt-locking mechanism that expands or contracts the space between the inside surfaces of the support-plate sections 7 based on pivoting of the adjustment lever 14. In the example in the figure, the pressure member 13 is constructed by a nut 13 that is mounted on the other end section in the axial direction of the rod member 12.
The cam apparatus 15 is constructed by a combination of a drive cam 16 and a driven cam 17. The drive cam 16 has a circular ring shape as a whole and is provided with a center hole for the rod member 12 to be inserted through. In the assembled state, a drive-side cam surface 20 is provided on the inside surface in the axial direction (right-side surface in FIG. 14) of the drive cam 16, and this drive-side cam surface 20 has plural drive-side concave surfaces 18 and plural drive-side convex surface 19 that protrude further inward in the width direction (right side in FIG. 14, and surface side in FIG. 16) than the drive-side concave surface 18, with these concave surfaces 18 and convex surface 19 being alternately arranged in the circumferential direction. The drive cam 16 is joined and fastened to the base-end section of the adjustment lever 14, and rotates reciprocatingly around the rod member 12 as the adjustment lever 14 is pivotally rotated. The drive cam 16 is provided in a state so as to be able to rotate relative to the rod member, or so as to be able to rotate in synchronization with the rod member 12.
The driven cam 17, similar to the drive cam 16, has a circular ring shape as a whole and is provided with a center hole for the rod member 12 to be inserted through. Moreover, on the outside surface (left-side surface in FIG. 14) in the width direction of the driven cam 17 in the assembled state, a driven-side cam surface 23 is provided on which plural driven-side concave surfaces 21 and plural driven-side convex surfaces 22 that protrude further outward in the width direction (left side in FIG. 14, surface side in FIG. 15) than the driven-side concave surfaces 21 are alternately arranged in the circumferential direction. One end in the circumferential direction of the driven side convex surfaces 22 (end section on the starting-point side of rotation when the drive cam 16 rotates according to the operation of the adjustment lever 14 for maintaining the height position of the steering wheel after adjustment, or in other words, the end section in the clockwise direction in FIG. 15), and the other end in the circumferential direction of the driven-side concave surfaces 21 (end section on the ending-point side of rotation when the drive cam 16 rotates according to the operation of the adjustment lever 14 for maintaining the height position of the steering wheel after adjustment, or in other words, the end section in the counterclockwise direction in FIG. 15) are continuous in the circumferential direction by way of driven-side continuous surfaces 24. Moreover, an engaging convex section 25 is formed on the inside surface in the width direction of the driven cam 17.
The drive cam 17 fits around the rod member 12 so as to be able to rotate relative to the rod member 12 and so as to be able to displace in the axial direction relative to the rod member 12. The engaging convex section 25 of the driven cam 17 engages inside the long tilt hole 8 in one of the support-plate sections 7 (left support-plate section 7 in FIG. 14) so as to be able to displace only along the long tilt hole 8. Therefore, the driven cam 17 can be raised or lowered along the long tilt hole 8, however, the driven cam 17 cannot rotate around its own axis.
To set a state in which the height position of the steering wheel can be adjusted, the adjustment lever 14 is rotated in a specified direction (typically downward). Then, as illustrated in FIG. 14, by setting the drive-side cam surface 20 and the driven-side cam surface 23 to a state where the drive-side convex surfaces 19 face the driven-side concave surfaces 21 and the drive-side concave surfaces 18 face the driven-side convex surfaces 22, the dimension in the axial direction of the cam apparatus 15 is reduced, and the space between the driven cam 17 and the pressure member 13 is increased. As a result, the surface pressure at the areas of contact between the inside surface of the support plate sections 7 and the outside surfaces of the supported-plate sections 10 is reduced or lost. In this state, it becomes possible to adjust the up-down position of the steering wheel within the range that the rod member 12 is able to move inside the long tilt hole 8.
In order to maintain the height position of the steering wheel after adjustment, the adjustment lever 14 is pivotally rotated in the opposite direction (typically upward) after the steering wheel has been moved to the desired height position. The drive-side cam surface 20 and driven-side cam surface 23 are set in a state such that the drive-side convex surfaces 19 come in contact with the driven-side convex surfaces 22, which increases the dimension in the axial direction of the cam apparatus 15, and reduces the space between the inside surfaces of the support-plate sections 7. In this state, the surface pressure at the areas of contact between the inside surfaces of the support-plate sections 7 and the outside surfaces of the supported-plate sections 10 increases, making it possible to maintain the height position of the steering wheel after adjustment.
In this kind of tilt steering apparatus, operation for setting the state for adjusting the height position of the steering wheel, or operation for setting the state for maintaining the height position of the steering wheel after adjustment is performed manually using the adjustment lever 14. The operation torque that is applied to the adjustment lever 14 for maintaining the height position of the steering wheel after adjustment tends to be greater than the operation torque applied to the adjustment lever 14 for setting the state for adjusting the height position of the steering wheel. Moreover, the operation torque that is applied to the adjustment lever 14 for maintaining the height position of the steering wheel after adjustment tends to increases going toward the contact state between the drive-side convex surfaces 19 of the drive-side cam surface 20 and the driven-side convex surfaces 22 of the driven-side cam surface 23 (closer to the latter half of operation of the adjustment lever 14). In a tilt steering apparatus, there is a need to improve operability by reducing this operation torque.
Conventionally, the driven-side continuous surfaces 24 of the driven-side cam surface 23 of the driven cam 17 is constructed by inclined surfaces 26a that are provided in the portions near the driven-side concave surfaces 21 (on one side in the circumferential direction), and inclined surfaces 26b that are provided on the other side in the circumferential direction from the inclines surfaces 26a. The angle of inclination of the inclined surfaces 26b with respect to an imaginary plane that exists in a direction that is orthogonal to the center axis of the driven cam 17 is less than the angle of inclination of the inclined surfaces 26a with respect to that imaginary plane. With this construction, an increase in operation torque that is necessary in the latter half of the operation of the adjustment lever 14 is suppressed. However, in this conventional construction, when the drive-side convex surfaces 19 of the drive-side cam surface 20 displace from the state of facing the driven-side convex surfaces 21 of the driven-side cam surface 23 up the driven-side continuous surfaces 24 of the driven-side cam surface 23 to the state of being in contact with the driven-side convex surfaces 22, the moment that is expressed as the product of the friction resistance at the areas of contact (friction areas) between the drive-side cam surface 20 and the driven-side cam surface 23 and the radius of rotation of the friction areas increases. Particularly, when the drive-side convex surfaces 19 ride up the inclined surfaces 26a, this moment suddenly increases and the operation torque that is necessary for operating the adjustment lever 19 also suddenly increases. Moreover, as the drive-side convex surfaces 19 of the drive-side cam surface 20 move up the driven-side continuous surfaces 24, the operational feel when operating the adjustment lever 14 may change at the boundaries with the inclined surfaces 26a and inclined surfaces 26b. As a result, the operator that is operating the adjustment lever 14 may feel that something is wrong.